The use of films to control the levels of reflection and transmission of a window at different frequency ranges of light is known in the art. For vehicle windows and many windows of buildings and residences, glare is reduced by controlling transmissivity of visible light (TVIS) and reflectivity of visible light (RVIS) at wavelengths between 400 nm and 700 nm. For the same window applications, heat load may be reduced by partially blocking solar transmission (TSOL) in one or both of the visible portion of the solar spectrum and the near infrared (700 nm to 1200 nm) portion.
One known sequence of films for providing solar control is shown in FIG. 1 and is described in U.S. Pat. No. 6,034,813 to Woodard et al., which is assigned to the assignee of the present invention. In FIG. 1, a solar control arrangement of films is attached to a glass substrate 12 by a pressure sensitive adhesive (PSA) 14. Originally, the solar control arrangement is formed on a flexible polyethylene terephthalate (PET) substrate 16. The solar control arrangement includes a Fabry-Perot interference filter 18, an adhesive layer 20, a gray metal layer 22, another PET substrate 24, and a hardcoat layer 26. The second adhesive layer 20 is used when the Fabry-Perot interference filter 18 is formed on one PET substrate 16, while the gray metal layer 22 is formed on the second PET substrate 24.
The Fabry-Perot interference filter 18 provides solar load reduction by preferentially passing light at certain wavelengths and reflecting light at other wavelengths. An example of a Fabry-Perot interference filter is described in U.S. Pat. No. 4,799,745 to Meyer et al. This patent describes a virtually transparent, infrared reflecting Fabry-Perot interference filter that is characterized by transparent metal layers spaced apart by dielectric layers of a metal oxide. The gray metal layer 22 of FIG. 1 contributes to the final optical properties of the arrangement. The Woodard et al. patent states that the gray metal layer is preferably formed of a metal or alloy, such as nickel chromium having a thickness in the range of 2 nm to 20 nm. The gray metal layer should be sufficiently thick to partially block the transmission of visible light through the film.
Another known optical arrangement is described in U.S. Pat. No. 6,707,610 to Woodard et al., which is also assigned to the assignee of the present invention. With reference to FIG. 2, an optical arrangement is shown as being adhered to glass 28 by a PSA 30. For example, the glass may be a windshield of a vehicle or a window of a building or home. The PSA layer 30 is sandwiched between the glass and a first PET substrate 32. On the opposite side of the PET substrate is a slip layer 34. An optical coating of titanium nitride has a thickness selected primarily for achieving desired optical characteristics, such as solar control. A nickel chromium layer 38 is described as being a damage-retardation layer. Rather than nickel chromium, other gray metal materials may be used. Atop the titanium nitride layer 36 is a laminating adhesive 40, a second PET substrate 42, and one or more protective layers 44, such as a hardcoat or anti-scratch layer.
In the design of optical arrangements for windows, optical considerations and structural considerations must be addressed. Tailoring transmissivity and reflectivity on the basis of wavelength provides advantages. For example, it is typically beneficial to have higher reflectivity in the infrared range than in the visible range of the spectrum. Within the visible range, color neutrality is often desired. Color neutrality should not vary with the angle of view and should not change with age. Regarding structural stability, reducing the susceptibility of coatings to cracking during fabrication, installation, or long-term use is an important consideration. During fabrication, films are exposed to high temperatures and pressures. During installation, cracks may develop as a consequence of bending, such as when a flexible coated PET substrate is bent to follow the contour of a windshield. When a coated polymeric substrate having a titanium nitride layer is flexed, the titanium nitride layer has a tendency to crack.
While the prior art approaches operate well for their intended purpose, further advances are sought.